A variety of commercial coating compositions are known for preparing metal surfaces to prevent corrosion and improve adhesion of paints or other coatings to the surface. For example, conversion coatings are used in commercial applications to provide anticorrosion properties to metal surfaces and/or to prepare the metal surfaces prior to painting operations. However, some conversion coatings, such as silane conversion coatings are colorless, are applied at very low coating weights, and are extremely difficult to detect by human visual inspection or colorimetric measuring devices.
Methods for determining the presence of thin films on a substrate are known. Certain fluorescent dyes in metal film coatings have been used to determine whether or not the coating has been applied to the surface. Specifically, the fluorescent brightening agents, such as stilbene and coumarin, have been added to chrome-free metal coatings. The fluorescent brightening agents allow for ultraviolet (UV) viewing of the coated metal under a UV light and detect the presence of the conversion coating by eye. These methods are useful in their ability to determine whether a coating has been applied to a substrate and to provide qualitative determination of the thickness of the coating, an important quality control matter.
Unfortunately however, the use of stilbene and coumarin to provide for quantitative determinations of coating thickness is not preferred because these compounds often do not display the required precision when they are used in a calibrated system to determine coating thickness. High precision is required in a commercially feasible coating thickness measurement system. The native fluorescence of the coating can interfere with measurement of fluorescent intensity of the coating.
In addition, many organic dyes are not readily soluble in waterborne conversion coatings, and thus, often affect the stability of the conversion coating. Addition of these organic dyes to conversion coatings results in precipitation or gelation of the other components of the conversion coating, thereby making the conversion coating composition unstable. The poor shelf-life of the conversion coating affects the usability of these conversion coatings because the precipitation or gelation results in non-uniform coating or the formation of voids or poorly coated surfaces. Poor corrosion resistance and poor adhesion of paints or other coatings to the metal substrate results when these unstable conversion coatings are used. Accordingly, there is a need to be able to quickly detect the presence, uniformity and thickness of these conversion coatings as an essential aspect of quality control, while avoiding the problems noted above with many organic dyes.